U.S. Pat. No. 4,973,223 describes a known variable geometry turbine in which a turbine wheel is mounted to rotate about a pre-determined axis within a housing. A sidewall is displaceable relative to a surface defined by the housing in order to control the width of gas flow passage defined adjacent the wheel between the sidewall and that surface. The sidewall is supported on rods extending parallel to the wheel rotation axis, and the rods are axially displaced relative to the housing so as to control the position adopted by the sidewall.
The rods are displaced by a pneumatic actuator mounted on the outside of the housing, the pneumatic actuator driving a piston which is displaceable parallel to the turbine axis. The actuator piston is coupled to the sidewall by a yoke pivotally supported on a bracket mounted on the housing, the yoke defining two spaced apart arms which extend on opposite sides of the turbine axis to engage portions of the support rods extending outside the housing. The end of each arm is received in a slot in a respective sidewall support rod. Displacement of the actuator piston causes the yoke to pivot and to drive the sidewall in the axial direction as a result of the interengagement between the yoke arms and the sidewall support rods.
In the known variable geometry turbine, the yoke pivot is located in the hostile environment outside the housing and cannot be readily lubricated. The engagement of the yoke arms with the rods is of a sliding nature and, although it is known to incorporate wear resistant relatively sliding surfaces made from for example ceramics, those surfaces cannot readily be lubricated. Accordingly wear can be a problem with the known assembly.
U.S. Pat. No. 5,522,697 describes an alternative yoke assembly to that described in U.S. Pat. No. 4,973,223. In that alternative assembly, the sidewall support rods are engaged by a yoke pivotally mounted within the housing on a shaft that extends outside the housing. An external actuator controls the rotation of the shaft and thus displacement of the yoke which engages in slots in the sidewall support rods. The yoke is mounted in a cavity immediately behind the sidewall.
With the arrangement of U.S. Pat. No. 5,522,697, the yoke is relatively compact and the yoke pivot and support rod engagement surfaces are located within the housing and therefore isolated from the hostile environment outside the housing. Unfortunately however the yoke is exposed to the conditions prevailing immediately behind the sidewall and it is not possible to lubricate the yoke given those conditions. As a result wear can still be a problem.